Electronic tachometer utilizing tuned signal transducer



ELECTRONIC TACHOMETER UTILIZING TUNED SIGNAL TRANSDUCER Filed Nov. 22.1960 R. L. REINER 3 Sheets-Sheet 1 Aug. 23, 1966 R. REINER 3,268,810

ELECTRONIC TACHOMETER UTILIZING TUNED SIGNAL TRANSDUCER "mn-21PM UnitedStates Patent O 3,268,816 ELECTRNIC TACHMETER U'HLEZING TUNED SHGNALTRANSDUCER Robert L. Reiner, West Caldwell, Nd. (Glenwood Court,Bethany, Conn.) Filed Nov. 22, 1960, Ser. No. 70,991 6 Claims. (Cl.324-70) My invention relates to tachometers, particularly to electronictachometers, and has for its primary object to provide an improvedelectronic circuit for measuring the number of revolutions per minute ofignition systems ignited by spark plugs.

The need for a tachometer which would not require special calibration orknowledge of the number of cylinders in an engine when used with varioustypes of ignition systems and particularly with respect to two cycle orfour cycle ignition systems has resulted in the present invention whichsamples an electrical condition common to all ignition systems. Myinvention is unique in that I take advantage of the low impedanceproduced during the arc discharge to drive the signal impulse andthereby actuate or trigger the subsequent circuitry to result in areading proportional to r.p.m. I, therefore, use the low impedance ofthe arc which is a reasonably constant parameter common to all ignitionsystems. Also, l sense this parameter electromagnetically through onlyone spark plug lead of the total number of spark plug leads required inan ignition system, so that the number of cylinders in the ignitionsystem is immaterial.

In order to utilize this arc discharge for all types of ignitionsystems, including magneto, battery and electronic, it is necessary tosense the electrical force which sustains the arc. In this invention itis the sustaining current that maintains the arc, which is utilized as asignal source at a sensing transducer. In addition to the currentassociated with the arc or spark there is always an associated rapidrise or fall of potential. Consequently, to insure overall systemstability it is necessary to isolate potential changes generated fromother spark plugs in order to prevent their appearance as signals, andthereby when calibrated for two cycle operation enable my tachometer tobe used on all two cycle engines regardless of the number of cylindersand, similarly, when calibrated for four cycle engines. This isimportant, since among the various ignition systems and during normaloperation of a gasoline engine this change in potential varies over verywide limits, such that it would be impractical to try to use it as ameans of signalling from one spark plug. Further, I avoid sensingpotential changes at the spark plug wire which is necessarily done at avery high impedance level and which is consequently very sensitive tostray electrostatic pick-up particularly from other spark plugs or otherignition systems which may be nearby.

Also, when sensing electrostatic changes as a signalling means inconventional tachometers, capacitive coupling is usually used to producea driving signal. In practice, any coupling capacitance in the signalcircuit has been found to contribute to unsteady meter indicationbecause of charging and discharging time constants. This occursprimarily during transient periods, either during changes in r.p.m. orloading on the gasoline engine. However, when sensing the current, as inmy invention capacitive coupling is eliminated. My arrangement iseiective during the arc discharge which is during a short circuitedcondition of the ignition system and, therefore, during a very lowdriving point impedance interval. The advantage of such sensing isnegligible susceptibility to stray electrostatic pick-up and, by properdesign of the sensing transducer, negligible loading at the ignitionsystem. The transducer, which senses these current impulses at a sparkplug and produces output signal pulses directly proportional to ther.p.m. of the engine irrespective of the number of cylinders, is themost important component in the entire tachometer system. It wasnecessary in the design of the transducer to isolate the sensing coilfrom the signal coil, so that current produced at the signal coil by thecurrent ilowing through the sensing coil during an arc at the spark plugwould dominate all other system noise, particularly, electrostatic.

It is therefore a further object of my invention to provide an improvedtachometer that is unaffected by potential variations in an ignitionsystem tired by spark plugs.

It is another object 0f my invention to provide an improved tachometerthat is stable over a Wide range of temperatures and that requires onlyone calibration when used with either two cycle or four cycle ignitionsystems.

It is still another object of my invention to provide an improvedelectronic tachometer that is simple in construction, reliable inoperation and is economically manufactured.

It is a further object of my invention to provide an irnprovedtachometer that maintains calibration accuracy over a wide input voltageswing.

Other and further objects will be obvious upon an understanding of theillustrative embodiment Vabout to be described, or will be indicated inthe appended claims, and Various advantages not referred to herein willoccur to one skilled in the art upon employment of the invention inpractice.

In accordance with the present invention the foregoing objects aregenerally accomplished by providing an irnproved universal tachometercomprising a transistorized voltage pulse counter which is stabilizedand regulated electrically, and a transducer which senses current pulsesin a spark plug lead of a gasoline engine. The transducer, which may belocated in the engine compartment and connected directly to the sparkplug wire, senses each time the spark plug fires, advantage being takenof the low impedance produced during the arc discharge, and transmits asignal pulse by a shielded cable to the counter, which may be located ona dashboard of a vehicle or bulkhead of a boat and which converts thesepulses into a steady meter reading with a linear scale calibrated inr.p.m.

A more thorough understanding of my invention may be obtained from astudy of the following description of several specific embodiments. Inthe drawings:

FIG. l is a schematic diagram showing a signal transducer of myinvention connected to a typical ignition system tired by a plurality ofspark plugs.

FIG. 2 illustrates typical wave forms of an ignition system supplying amulti-cylinder engine through a distributor having a primary drivingforce of coil, magneto or electronic.

F'IG. 3 illustrates typical wave forms of output voltage lat the 'output:terminals of fthe signal transducer.

FIG. 4 is a schematic dnawing showing la counter circuit and indicatingcircuit of my invention, adapted tto be connected to the signalmnansducer of FIG. 1 without capacitive coupling.

FIG. 5 is la :schematic `diagram -of a composite tachometer circuitincluding complete signal transducer, eonnter and indicating circuits inits simplest form.

Referring now Ito fthe idnawings, panticularly tno IFIG. l, there showna convencional ignition system having a storage battery 20, primary coil21, secondary coil 22, breaker points 23 with timer-cam 24, whichnapidly makes and lbreaks the circuit between the breaker points 23,`and la condenser 26 which shunts the lbreaker points 23. A lead 27connects ia voltage potential fnom the secondary coil 22 tto la notarylarm 28 of la distributor 29 Iwhich includes terminals 3*] 'over whichthe contact farm 28 rides, as is well known. Each of the terminals 31 isconnected 3 to la spark plug I32 for each cylinder (not shown), so that'the ignition system shown is for a four cylinder engine.

In the present invention connection lof ya signal rtransfducer 3'3 isreadily made to the ignition system either at the spark plug or thedistributor, -or by cutting the high 'voltage wire 27 along its length,so that the current owing in fthe wire, when the spark plug 62 tires,will iiow through .the signal transducer 3'3. As seen in iFlG. 1, thesignal transducer 33 comprises a transformer 34 having a primary Windingor sensing coil 66, bridged by xa condenser 37 (which provides ra lowimpedance), and ra secondary winding lor signal coil 38. The primarywinding or sensing coil 36 is also bridged respectively to distributtorterminal 31 :and `spark plug 32, say for lthe ,first cylinder of theengine, so that the signal transducer 33 is serially interposed betweenthe distributor rand the spark plug.

In order to minimize electrostatic impulses at the output of thetransducer I33, 'the sensing coil 36 and the signal coil 38 are soconstructed that the sensing coil 36 is physically separated from thesignal coil 38 lby sufficient physical distance :to keep theelectrostatic coupling to ra very low value. This separation also helpsprevent voltage breakdown. In a prototype of my invention the corematerial was constituted by 'an inexpensive ferrite rod (not shown)approximately three inches long; however, iron core material has beenfound to Ioperate satisfactorily. In order to further reduceelectrostatic sensitivity, the condenser 37 is bridged across thesensing coil `36, whereby electrostatic charging currents liowingbetween the spark plug 32 and ground are `by-passed and cannot beconfused as a true signal impulse. This condenser 37, during the lowimpedance arc discharge, has little shunt-ing effect of the currentproduced thereby 'and a strong current signal is Ilthereforemagnetically coupled between the sensing winding 36 and the signalwinding 38 of the signal transducer 33. In practice the signaltransducer 33 is remote from the rest of the tachometer circuitry, sinceit is convendent to make connections at the distributor lor spark pluglead Within the engine cowl and locate the tachomete-r indicator andcircuitry on the dashboard for other par-t of the boat or automobile,etc. The signal from the tra-nsducer 33 is connected by -a shieldedcable lor twisted pair comprising leads 39 and 40 to a counter circuit41, FIG. 4, in order to minimize electrostatic noise. It might bementioned at this point that my tachometer comprises fou-r distinctparts-a signal transducer 33; an interconnecting cable comprising leads39 and 40; a counter circuit 41; and a visu-al indicating circuit 42.

Referring now to IF IG. 4, the counter circuit 41 is shown as beingbased upon the properties of a driven blocking oscillator in which atransistor 43 is used las its active element rather than la vacuum tube.The principle by which n controlled pulse is produced is generallylnnown by those versed 'in the art and can be briefly described asfollows: transistor 43 |has la base b, which is .a control electrodecorresponding to 'the grid of ya vacuum tube, 'an emitter e whichcorresponds to the cathode and la collector c which corresponds Ito theplate of a vacuum tube. These elements form, in effect, switchingcontacts such that for an NPN transistor a positive current flow betweenfthe base b and the emitter e causes the collector-emitter contact toclose allowing .a large signal to ilow. Similarly, the Lack of apositive current flow between base b and emitter e causes thecollector-emitter contact to open, whereby no signal current can flow.This, of course, is the ideal case. in practice, however, the transistorA43 is never fully an cpen circuit nor fully a short circuit. However,[by driving the base b with ra large positive current the transistor 43can be made to 'approach la short circuit; rand `by applying a reversecurrent between base b `and emitter e, in other words making the base bnegative with respect to fthe emitter e, the transistor 43 can be madeto approach an open circuit.

In this counter circuit 41, a diode `44, preferably of 4 silicon, isused lto provide the reverse bias. Initially no current iiows from thepositive terminal `of battery 20 through .the indicating circuit 42 andwinding 46 of transformer 47, since the transistor 43 is assumed open.If a -sma-ll positive current is now made to iiow, say from thetransducer 33 through the base b of transistor 43, a voltage drop willbe produced across 'the winding 46 of transformer 47, because thecollector c is being driven towards the emitter e by the small current.This drop is coupled to winding 48 by transformer notion =and the dropacross winding 48 is in such -a direction as to reinforce the originalpositive current iiow through the transistor base b, thereby making thecurrent cumulative. Transistor 413 is then rapidly driven to saturation,so that the collector emitter contacts can now be considered closed. Thefull battery voltage .less the drop across diode 44 and condenser 45 nowappears across winding 46 of transformer 47, thus causing a magnetizingcurrent to iiow, which current is proportional to the primary inductanceof the transformer 47. The vlengt-h of time that the transistor 43remains saturated or closed is determined by the time that themagnetizing current continues Ito build up in the core yof transforme-r47. This, then, is the length of time for a unit energy impulse to flowthrough the indicating circuit 42. It can be shown that 'variations inbattery supply volt-age will shorten lor lengthen the time of thisimpulse resulting in la constant energy situation regardless of batterysupply voltage.

After this forward pulse of energy terminates, because of saturation oflthe core transformer 47, an amount of power, proportional to the volumeof the core material of transformer 47, is released and produces apotential drop across both windings 46 and 48 of wha-tever magnitude isnecessary to cause a discharging current to flow. This is the commoninductive kick principle associated with the discharge of any magneticcore and, in all blocking oscillator circuits, can develop high voltagesacross the transistor 43. If these voltages are no-t properly controlledor limited, the transistor 43 can be ruined. Consequently, protectionlof transistor 43 is aorded through diodes 49 and 50, the first of whichlimits the reverse polarity of winding 48 t-o a few volts and the secondof which provides an additional discharge path, as will be describedhereinafter. A smal-l resistor 51, not usually required, can be includedyas shown to speed discharge of the magnetic core and its subsequentreset to allow the cycle to repeat. Diode 49, in addition to providingtransistor protection, has a primary function `of transferring positivesignal impulses from the signal winding 38 of transducer 33 to the baseb of transistor 43 through a current limiting resistor 52. Once a strongenough positive signal is supplied from the signal Winding 38 of thetransducer 33 through diode 49 to the base b circuit of transistor 43,regenerative action takes place for driving transistor 43 full on. Theupper end, as viewed in FIG. 4, of winding 48 is driven several voltspositive Ito insure su'icient base b current flow in transistor 43.Diode 49 is then also used to decouple the signa-l winding 38 of thetransducer 33, so that current produced by ythe positive drop acrosswinding 48 flows through the base b circuit of transistor 43 rather thanacross the signal winding 3S of the transducer 33.

After a unit energy impulse has terminated, diode 49, as explainedhereinbefore, and diode 50 are used to complete discharge paths for Atheenergy lcontained in the core of transformer 47. The rst discharge pathmay be traced from winding 48 of transformer 47 via lead 40, throughwinding 38 of transducer 33, over lead 39, diode 49, resistor 51 andback through Winding 48. The other discharge path is from winding 48,over lead 40, diode 50 and` thence Via resistance 51 back throughwinding 48. In general, negative potential can be considered ground orearth; however, for the system to operate properly either batterypolarity may be grounded and, if more convenient, the entire tachometercircuit can be left oating.

The indicating circuit 42 comprises a direct current sensing ammeter 53and an integrating network of sufciently long time constant to smooth arepetition of unit energy impulses into a steady direct current flowthrough the ammeter 53. Many configurations for producing this arepossible, only one of which is shown and which comprises a resistor 54in series with lthe ammeter 53 and a Variable resistor 56 and condenser45 each in parallel with said ammeter 53 and resistor 54.

The operation of the circuit will now be considered. Each time upondischarge of the selected spark plug (No. 1) a signal is produced whichcomprises current ow through the vsignal transducer sensing winding 36with a corresponding Voltage drop being produced therein and also in thesignal winding 38. The signal is then coupled via leads 39 and 40 to thecounter circuit 41, as described earlier, resulting in a unit of energyiiow through the indicating circuit 42. The initial polarity of theVoltage drop at the signal winding 38, because of the direction of thecurrent flow ithrough the sensing winding 36, is unimportant, sincetransformer 34 and condenser 37 form a high Q (quality) ringing circuitwhich helps the normally oscillatory arc current. Assuming that theinitial signal applied to diode 49 is a negative going signal, it willbe blocked by diode action of diode 49 and no appreci- `able currentwill flow until the signal oscillates to a positive going polarity whichis proper signal polarity for triggering the counter circuit 41 intoconduction. Condenser 37 forms a low impedance path, as describedearlier, to by-pass electrostatic changing currents, so that a -dropcaused thereby does not appreciably appear across the sensing winding 36of the transducer 33. Because in a practical signal transducer andassociated cable electrostatic voltage transients generated by otherspark plugs in the ignition system `cannot be entirely eliminated, anunwanted signal is coupled through diode 49 to tnansistor 43 and thisunwanted signal must be kept below a minimu-m threshold, as seen in FIG.3, so that it cannot trigger the counter circuit into conduction. Thisis important, since such signals are not of consistently high amplitudeand cannot be included in the calibration of even a single ignitionsystem.

In this connection, condenser 57 with resistor' 52 forms an integratingnetwork of such time constant that these unwanted signals are reduced,as seen in FIG. 3, below the discrimination level afforded by diode 44and therefore cannot trigger the circuit. Resistor 52 serves twoadditional purposes. It limits the peak positive drive current throughthe base b of the transistor 43, as explained earlier, and also withlresistor 51 provides a collector circuit load impedance which isree-cted into winding 46 by transformer action. As long as resistor 52is made large with respect to the variations in the base b resistance oftransistor 43 over a specific operating temperture range, the stabilityand accuracy of the entire circuit can be predictably controlled and nospecial compensation is required. In general, a value for resistor 52and, by proper choice of primary inductance of transformer 47, pulsewidth can be selected to produce optimal transistor base drive andcollector load, so that a practical meter operating current is produced.In general this may be one milli-ampere for full scale deection toreiiect a two cycle or four cycle r.p.m. rate of perhaps 6,000 or 8,000resulting in a signal repetition rate from 3,000 to 8,000 pulses persecond for full scale readings. After resistor 52 is chosen, -condenser57 is picked to produce the desired intergrating time constant.

For each signal of positive going polarity applied to the transistor 43,closure of the latter sends a similar pulse through the indicatingcircuit 42. Each such pulse may be traced from positive potential ofbattery over leads 59, 61, via meter 53, resistor 54, lead 62, winding46, lead 63, closed collector-emitter Contact, lead 64, diode 44, andthence back over leads 66, 40 and 67 to negative terminal of battery 20.Thus the series `of pulses produced by the spark plug and illustrated inFIG. 2, are translated by the transducer 34 into a series of closures ofthe transistor 43, which in turn produces a series of repetitive pulsesthat, as explained hereinbefore, are integrated into a steady reading atthe meter 53, which reading is calibrated by resistance 56 intocorresponding r.p.m. of the engine. To produce suiiicient current flowthrough the diode 44 in the above-traced circuit, such that its inherentvoltage drop of about 0.7 volt for a silicon device is achieved, aresistor 65 is connected at one end to the positive side of the battery20 and at its other end to lead 64 between the emitter e contact anddiode 44. If this resistor 65 is eliminated, the voltage drop acrossdiode 44 because of leakage current flow in transistor 43 may not besufficient to reverse bias the transistor at elevated temperatures withthe consequence that the circuit will -become a free running oscillatorat these elevated temperatures.

In order to provide economical battery supply operation the function ofthe diode 44 of FIG. 4 can be replaced by a single battery cell 68, asseen in FIG. 5, and the resistor 65 of FIG. 4 eliminated. It isnecessary that the battery 68 have a fairly constant voltage drop overits entire operating life, since variations in the bias voltage willdirectly affect accuracy. The current demand from the battery supply 68during stand-by will not exceed the very small leakage current ilow intransistor 43.

It is practical to operate my tachometer from an external battery havingvery low internal impedance, such as a 6 volt or 12 volt lead-acidstorage battery 20. If dry cell batteries 69, as seen in FIG. 5, areemployed for the power supply, a shunting condenser 71 is provided inorder to insure the necessary low supply impedance, so that calibrationaccuracy can be maintained throughout the discharge life of the battery69.

Referring again to FIG. 5, which illustrates simplified counter andindicating circuits 41 and 42', respectively, combined with thetransducer circuit 33, it will be noted that the resistors 51 and 52have been replaced by a variable resistor 72 and the diode 50eliminated. The operation of FIG. 5 is otherwise substantially thatdescribed 4in connection with FIG. 4.

An example of component part Values for a representative embodiment ofFIGS. 1 and 4, exclusive of ignition circuit, is as follows:

Condensers:

37 microfarad 0.02 45 do 400 57 do 0.01

Diodes:

44 1N483 49 1N805 50 IN805 Resistors:

51 ohrns-- 100 52 do 1,500 54 do 1,000 56 do 500 do 5,600 Transistor:

43 (NPN type) 2N1059 Meter:

53 ma. F.S 0-1 Transformers:

34, ferrite core- Winding 36 turns 250 Winding 38 do 150 47, saturablecore- Winding 46 turns 250 Winding 48 do 500 The Values listedhereinbefore are for a particular ernbodiment of the invention and arenot to be `considered as limiting the invention. Also, it is to beunderstood that any other type of switching means provided by the NPNtype junction transistor might be substituted therefor Without departingfrom this invention. As Various changes may be made in the form,construction, and arrangement of the parts herein, without departingfrom the spirit and scope of the invention and without sacricing any ofits advantages, it is to be understood that all matters are to beinterpreted as illustrative and not in any limiting sense.

What is claimed is:

1. In an electronic tachometer for an engine having an ignition circuitired by a spark plug, a sensing coil connected serially to said sparkplug in a high voltage circuit of said ignition circuit, a condenserconnected in parallel with said sensing coil for providing a lowimpedance path for current discharges of said spark plug and forproviding a time integral of each of said current discharges, wherebythe current discharges are transformed into a series of constant peakamplitude signal voltages, an elongated conductive rod carrying saidsensing coil at one end thereof, a signalling coil carried by said rodat its other end, whereby said sensing coil and said signaling coil arephysically separated from each other so as to avoid arcing, saidsignaling coil being effective to isolate effects of said noise currentsand to transform said constant peak amplitude signal voltages, saidsensing and signaling coils being solely electromagnetically coupled, atuned counter circuit responsive to said transformed signal voltages,said counter circuit including transistor switching means, saidtransistor switching means including an emitter, collector and baseconnections, and a meter circuit responsive to said counter circuit forindicating said spark plug discharges and correspondingly the speed ofsaid engine.

2. In an electronic tachometer according to claim 1, wherein saidsensing coil serially connected with said spark plug senses eachdischarge yof the spark plug and said signaling coil signals each saiddischarge to the counter circuit, and a diode serially interposedbetween said signaling coil and said base connection of said transistorswitching means for providing positive current ow between said base andemitter connections `of said transistor switching means at each saiddischarge of said spark plug.

3. In an electronic tachometer according to claim 2, wherein a resistoris connected in series with said diode and base connection of saidtransistor switching means.

4. In an electronic tachometer according to claim 3, wherein a`condenser is connected at one terminal to said resistor and at itsother terminal to a terminal of said signaling coil to provide with saidresistor an integrating network for reducing unwanted signals.

5. In an electronic tachometer for an engine having an ignition circuitfired by a spark plug, a tuned transducer serially connected to saidspark plug in a high voltage circuit of said ignition circuit, saidtransducer including a capacitor and a sensing winding connected inparallel, said capacitor providing a low impedance path for currentdischarge of said spark plug and providing a time integral of saidcurrent discharge, whereby the current discharge is transformed into aconstant peak amplitude signal voltage, said capacitor also providing ashunt by-pass for electrostatically coupled noise currents from adjacentspark plug conductors of the ignition circuit, said transducer alsoincluding a signaling coil for isolating etects of said noise currentsand for transforming said constant peak amplitude signal voltage, saidsensing and signaling coils being solely electromagnetically coupled, atuned counter circuit responsive to said transformed signal voltage, andan indicating circuit responsive to said tuned counter circuit.

6. In an electronic tachometer according to claim 5, wherein said tunedcounter circuit includes filter means for accepting said transformedsignal voltage and for rejecting said effects of said noise currents.

References Cited by the Examiner UNITED STATES PATENTS 2,005,992 6/1935Heaton 324-18 2,226,185 12/ 1940 Sturm 324-70 2,312,840 3/1943 Lansdale324-16 2,485,666 10/1949 Silver 336-182 2,603,685 7/1952 Bychinski324-15 2,630,529 3/1953 Mann et al 324-70 2,810,080 10/ 1957 Trousdale331-112 X 2,817,058 12/1957 Weidner 324-16 2,848,613 8/1958 Green et al331-112 2,857,518 10/1958 Reed 331-112 2,873,388 2/1959 Trumbo 324-78 X2,902,647 9/ 1959 Hartung 324-70 2,927,268 3/ 1960 Haggai et al. 324-702,934,703 4/1960 Cohen 324-78 X 2,936,383 5/1960 Mees 331-112 X2,962,654 11/1960 Wilson 324-16 X 3,010,032 11/1961 Carney 331-112 X3,056,084 9/1962 Parmater 324-70 3,094,656 6/1963 Miles 324-70 X3,104,329 9/1963 Haas 307-885 FOREIGN PATENTS 602,200 9/ 1934 Germany.

WALTER L. CARLSON, Primary Examiner.

S. BERNSTEIN, Examiner.

A. E. RICHMOND, Assistant Examiner.

5. IN AN ELECTRONIC TACHOMETER FOR AN ENGINE HAVING AN IGNITION CIRCUITFIRED BY A SPARK PLUG, A TUNED TRANSDUCER SERIALLY CONNECTED TO SAIDSPARK PLUG IN A HIGH VOLTAGE CIRCUIT OF SAID IGNITION CIRCUIT, SAIDTRANSDUCER INCLUDING A CAPACITOR AND A SENSING WINDING CONNECTED INPARALLEL, SAID CAPACITOR PROVIDING A LOW IMPEDANCE PATH FOR CURRENTDISCHARGE OF SAID SPARK PLUG AND PROVIDING A TIME INTEGRAL OF SAIDCURRENT DISCHARGE, WHEREBY THE CURRENT DISCHARGE IS TRANSFORMED INTO ACONSTANT PEAK AMPLITUDE SIGNAL VOLTAGE, SAID CAPACITOR ALSO PROVIDING ASHUNT BY-PASS FOR ELECTROSTATICALLY COUPLED NOISE CURRENTS FROM ADJACENTSPARK PLUG CONDUCTORS OF THE IGNITION CIRCUIT, SAID TRANSDUCER ALSOINCLUDING A SIGNALING COIL FOR ISOLATING EFFECTS OF SAID NOISE CURRENTSAND FOR TRANSFORMING SAID CONSTANT PEAK AMPLITUDE SIGNAL VOLTAGE, SAIDSENSING AND SIGNALING COILS BEING SOLELY ELECTROMAGNETICALLY COUPLED, ATUNED COUNTER CIRCUIT RESPONSIVE TO SAID TRANSFORMED SIGNAL VOLTAGE, ANDAN INDICATING CIRCUIT RESPONSIVE TO SAID TUNED COUNTER CIRCUIT.